The present invention resides in an improved lightweight, flame resistant composite panel which is readily shaped at room temperature without the necessity of machining the panel.
Composite panels having a core of compact thermoplastic material sandwiched between a pair of metallic covering layers are known in the art. U.S. Pat. No. 3,382,136 to Bugel et al. discloses a typical moldable composite panel. The composite panel is made up of a core of a compact (i.e. not foamed) thermoplastic material, preferably polyethylene, sandwiched between a pair of metallic covering layers. The resulting composite panel is relatively lightweight and may be shaped by bending or the like without the necessity of machining. In order for the composite panels to be shaped it is important that they exhibit a high specific stiffness, the specific stiffness being defined by the modules of elasticity of the metallic cover layer times the moment of inertia divided by the specific weight of the composite panel. The plastic shaping of the composite panel requires that the plastic core have enough strength, especially shear strength to prevent the buckling of the metallic cover layer when the composite is bent.
While the above-noted prior art composite panels may be readily formed at room temperature they suffer the disadvantage of being rather flammable. According to the U.S. Uniform Building Code (UBC), the composite panel of the prior art, such as Bugel et al., which has a core of compact polyethylene, is a Class III material. The National Fire Protection Association (NFPA) rates the combustion behavior of the same type composite panel as a Class C material. Thus, the composite panel is less fire resistant than each of its components. This may result from a number of reasons, one being that the metallic covering layers become heat conductors in the case of a fire and cause the thermoplastic to melt. The adhesive is then brought into contact with the air which is disastrous in the case of adhesives which are combustible. Because of the relatively poor combustion characteristics of known prior art composite panels their use as construction materials is considerably limited.
Accordingly, it is the principal object of the present invention to provide a composite panel characterized by improved fire resistant properties.
It is a particular object of the present invention to provide a composite panel which is lightweight and which is characterized by a high specific stiffness.
It is a still further object of the present invention to provide a composite panel which is readily shaped at room temperature without the need for machining.
Further objects and advantages of the present invention will appear hereinbelow.